Lab talk

Sep 28, 2012

Helium ion beam milling provides higher resolution patterning

Scientists have successfully fabricated a domain wall magneto-resistance spin valve with a bridge length of 32.8 nm and width of just 13.1 nm. The work has been published recently in the journal Nanotechnology, together with the device's step-like spin-valve resistance–external magnetic field curve. The final gap–bridge structure was formed using helium ion milling, which allowed the team to produce a device with a thinner gap than was possible with e-beam lithography alone. A smaller bridge width and gap distance leads to greater magnetoresistance. Applications for the spin-valves include future solid-state memory drives and other systems such as logic cells and sensors.

Helium ion microscopy is a surface imaging technique that involves scanning a focused helium ion beam across a sample surface. The image is formed from secondary electron emission in a similar way to a scanning electron microscope (SEM), but the larger mass and therefore shorter de Broglie wavelength of a helium ion compared with that of an electron leads to a smaller interaction volume and a therefore a higher resolution compared with an SEM.

Higher resolution patterning

The helium ion microscope (HIM) can also be used as a milling tool to remove material directly from a surface in a similar way to a gallium focused ion beam (FIB). The low mass of helium compared with gallium results in a much reduced sputtering yield and therefore lower milling rates, with the more tightly focused beam leading to higher resolution patterning.

In the study, the group from the University of Southampton, UK, fabricated constricted domain wall spin valves that show considerable magneto-resistance when the Ni pads are switched by a magnetic field from an anti-parallel to a parallel configuration.

The implantation of He ions leads to sub-surface swelling, which limits the process to the fabrication of sub-30 nm device features and makes the tool complementary to e-beam lithography.

Group details

The Southampton Nanofabrication Centre run by the Nano Research Group is a state-of-the-art facility for micro-fabrication and high-spec nanofabrication, as well as a wide range of characterization capabilities housed in the new Mountbatten Complex at the University of Southampton. The scientists in the Nano Research Group focus on fabrication and engineering from the micro-scale down to the nanometre scale to produce a wide range of novel devices, materials and integrated systems. Current research topics include nano-magnetic materials and devices, nano-photonics, graphene electronics, biosensors, MEMS/NEMS devices, solar cells, new materials, as well as continuing work on ultimate MOS devices.

About the author

The research was carried out by a team at the Nano Research Group in the Faculty of Physical and Applied Science at the University of Southampton, UK. Mr Yudong Wang is currently a postdoctoral research fellow funded by EPSRC at the University of Southampton. He is the electron beam lithography system operator and performed the full fabrication and characterization processes including theoretical modelling in this project supported by his PhD supervisor, Prof. C H “Kees” de Groot. Dr Stuart A Boden is a postdoctoral research fellow and operates the helium ion beam microscope and focused ion beam system. He performed the helium ion beam milling and simulations in this project. Kees de Groot is a professor in nanoelectronics and spintronics at the University of Southampton. Prof. Darren M Bagnall is the head of the Nano Research Group and contributed to the development of the milling process. Prof. Harvey N Rutt is a professor of Infrared Science and Technology and a consultant in helium ion microscopy including vibration and acoustic suppression of the machine environment.